The present invention relates generally to multilayer plastic containers, preforms and articles of manufacture, and more specifically to multilayer plastic containers and preforms that are resistant to delamination and provide a barrier to oxygen and other gases permeating through the layers of the container/preform wall.
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
It is well known in the art to provide thermoplastic packaging articles such as films, bottles, containers, and the like, which are useful for food and beverage packaging and which provide low oxygen permeability. Low oxygen permeability is important in the food & beverage industry because permeation of oxygen from air through the package can cause deterioration of the stored product—especially for the more highly oxygen-sensitive food & beverage products such as beer, orange juice, green tea and tomato-based products such as ketchups, salsas, pasta sauces and other processed food products.
In order to enhance freshness preservation, it is a standard practice to package food and beverage products within a packaging structure composed of laminated sheets of two or more plastics. Such packaging structures generally include at least one layer of an oxygen barrier material comprising a plastic which has a lower permeability to oxygen than the matrix polymer, which typically provides the structural and mechanical properties of the packaging structure. There are two general types of oxygen-barrier materials—passive and active. A “passive” barrier retards oxygen permeation into the package. For example, with multi-layer technology it is possible to incorporate thin layers of expensive barrier polymers (e.g., polyvinylidene chloride copolymer (PVDC), ethylene vinyl alcohol copolymer (EVOH), and poly-m-xylyleneadipamide (MXD6 or PA-MXD6)), in combination with structural layers of bottle-grade plastic resins (e.g., polyethylene terephthalate (PET)), to provide a cost-effective barrier package.
In an “active” barrier layer, an oxygen “scavenger” is incorporated into a single or multi-layer plastic structure to theoretically remove the oxygen initially present and/or generated from the inside of the package, as well as to retard the passage of exterior oxygen into the package. An example of an active barrier layer is the nylon MXD6 because it was discovered that MXD6 (as well as certain other nylons), in the presence of certain metals, reacts with molecular oxygen as it permeates through the wall of a container (see, e.g., U.S. Pat. No. 5,021,515). Thus, in addition to having good passive barrier properties, MXD6 is also employed as an active oxygen barrier layer in the presence of certain metals. Accordingly, MXD6 is widely used in the plastic packaging industry as either an active or a passive barrier layer in a multilayer container such as, for example, a multilayer PET container having the following configurations: PET/MXD6/PET or PET/MXD6/PET/MXD6/PET.
A problem with such PET/MXD6 multilayer containers is that the adhesive strength between the MXD6 layer and the layers of PET is weak. Thus, it is not uncommon for bottles formed with three or five layered constructions of PET and MXD6 or other nylons and nylon blends to delaminate due to the insufficient interlayer adhesion between adjacent layers, particularly after the preforms are stretch blow molded and filled with carbonated beverages under pressure or are hot filled. Such a delamination of the barrier material layer often increases after some storage time or moisture absorption, most likely due to the shrinkage caused by chain relaxation and post-crystallization of the barrier resin. The layer delamination in such bottles is further aggravated by impact stresses such as dropping the bottle from a height or impacting on the sidewalls. Stress concentration upon sudden impacts causes the initiation of layer delamination if the adhesion is poor.
The layer delamination problem in the multilayer PET bottles is undesirable not only for the retention of the structural integrity and aesthetic appearance of the bottle, but also for retaining its impact toughness and barrier performance. Accordingly, there is a need in the art for an adhesive-promoting material to be employed in conjunction with a barrier polymer such as, for example, MXD6 in the barrier layer and/or matrix layer of a multilayer plastic container/preform wall that is effective in increasing the interlaminar adhesion between the layers of different polymers in a multilayer plastic container to yield improved delamination resistance. Thus, it is an object of the present invention to provide a multilayer plastic container, preform or article of manufacture having suitable interlaminar adhesion between the several layers of the multilayer wall and having a high barrier against permeation of oxygen therethrough.
The present invention provides a solution to this practically significant problem in multilayer PET bottles by including a suitable thermoplastic compatibilizer additive to improve the adhesion between the PET and barrier polyamide layers. The adhesion-promoting thermoplastic additives of this invention have structural segments that are believed to be compatible with the PET and the barrier polyamide, bridging and strengthening the interface enough to cause high adhesion to PET in the multilayered containers.